This application claims the priority of German Application No. 198 30 234.7, filed Jul. 7, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for testing the tightness of a fuel tank in a motor vehicle, said tank having a ventilation system.
To diagnose leaks in a motor vehicle fuel tank, the tank is initially pressurized and then sealed pressure-tight. Next, the pressure in the tank is monitored for a certain time period. If the pressure does not change beyond a certain degree, then the system is assumed to be sealed tight. Such diagnostic measures are known for example from German Patent documents DE 44 27 688 A1, DE 4303 997 A1, and DE 196 36 713 A1.
In known methods, leaks usually can be detected only when they are 1 mm or more in diameter. Smaller leaks present problems in reliably detecting the leaks. Under existing legislation, in the United States for example, where leaks 0.5 mm in diameter must be detected, diagnostic procedures must be improved accordingly. In known methods, a pressure curve is recorded during diagnosis. However, a problem arises in that compensatory processes operate after the interior tank pressure has been lowered which cause the pressure to rise even in a tank that is absolutely tight. Such compensatory processes include outgassing or condensation of the fuel. The rise in pressure caused by such compensatory processes masks pressure changes produced by very small leaks and thus prevents a reliable diagnosis. When the compensatory processes (lasting approximately 10 to 15 minutes) are complete, a uniform state is established in which the effects of leaks can be distinguished more clearly. However, in the case of "on-board" tests, no time is available to wait for this state to become established.
In known methods, leak diagnosis can be performed only when the vehicle is at rest or operating in neutral and requires several minutes, for example when stopped at a traffic light. Other boundary conditions must be fulfilled as well. This operating state of the vehicle however is also regularly used to purge the container that absorbs fuel vapors. Such a container usually contains activated charcoal. This container must be flushed regularly and as often as possible, for which purpose a shutoff valve is opened and the flushing air is supplied to the internal combustion engine through a vent line and scavenging valve, which is also open. In this fashion, the activated charcoal filter is purged once again. If a tightness diagnosis is performed in these operating states, there is an inadequate amount of time to purge the activated charcoal container. Since incorrect measurements frequently occur with known diagnostic methods, the test must be repeated often until reliable values are obtained. This means that valuable time is wasted in purging the activated charcoal container with known methods, so that the container must be correspondingly generously dimensioned for safety reasons so that it remains operable at all times.
Therefore, the goal of the present invention is to improve the accuracy of the method mentioned at the outset using error diagnosis, with the test being performed as much as possible without interfering with normal driving, especially with purging of the activated charcoal container.
According to the invention, this goal is achieved by a method for testing a fuel tank in a motor vehicle for tightness, is with a container that absorbs fuel vapors, said container being connected through a vent line with a fuel tank and/or scavenging line with an intake manifold on the engine of the motor vehicle, and which has a vent line connected with the atmosphere, said line being closable by a shutoff valve, with a pressure sensor that measures system pressure in the fuel tank, and with a scavenging valve located in the scavenging line, said valve being opened to admit the fuel vapors stored in the container and to develop a vacuum in the tank venting system. The fuel tank is kept at least approximately during the entire operation of the motor vehicle at a specific pressure. After a preliminary period which at least approximately corresponds to the amount of time in which compensatory processes such as condensation or outgassing of fuel run to completion, a tightness test is performed at any point in time and in any operating state of the vehicle with the fuel tank sealed.
One of the important features of the invention is that the fuel tank is kept under a specific pressure for nearly the entire period of operation. This means that after a certain preliminary period during which the compensatory processes take place, stable operation prevails nearly throughout the entire operating time of the motor vehicle and a test can be performed at any opportunity, without being distorted by compensatory processes. One important advantage of the invention consists in the fact that one is no longer limited, as in the prior art, to performing the test with the vehicle at rest or in neutral, at which time the necessary purging of the activated charcoal container can also be performed. Rather, the assessing method according to the invention can also be performed with the vehicle in motion, for example with a partial load.
Advantageously, the fuel tank is operated under a slight vacuum since this can be created in a very simple fashion, for example by a suitable setting of the shutoff valve and designing the latter as a regulating valve.
Another advantage of operating the fuel tank under constant vacuum is that it further refines the testing of the fuel tank for tightness. This is because after the fuel tank has been tested for tightness, the fuel tank can be operated for a short time at zero pressure or with overpressure, with any possible outgassing or condensation being measured and subtracted from the values determined during the previous test.
The fact that the fuel tank is operated under a constant vacuum means that the compensatory processes will run to A completion after a certain time. If now, according to the invention, the fuel tank is nevertheless pressurized for a short time or if it is kept at least at zero pressure, it is possible to check in this fashion whether there was any outgassing or condensation during the previous measurement.
This would be the case, for example, if the fuel was relatively warm, since then the highly volatile components of the fuel could outgas. With the expansion according to the invention, the measurement accuracy can be improved even further in this manner.
In an additional improvement on the invention, provision can also be made to take the tank volume and the fullness of the tank into account during the test.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.